Another possibility is that it's a character from a science - fiction setting. Say an astronaut who was lost in space and now is back on Earth. The main character is usually the one whose return is the central focus of the fanfic and around whom the plot revolves.
I'm not sure specifically as it depends on the particular 'I'm back fanfic'. It could be a returning hero from a well - known story. For example, if it's a superhero fanfic, it might be a superhero who was thought to be dead or missing.
Most likely, Marty McFly would still be one of the main characters. He's the central figure in the original movies and would probably play a major role in the fanfic set in 2015. Doc Brown would also be there, as he's the inventor of the time - travel technology and Marty's mentor. There could also be Biff Tannen or his descendants, as they often cause trouble in the 'Back to the Future' universe.
Most likely Bella and Edward would still be main characters. Since it's a Twilight - based fanfic, their relationship is often central to the story.
Another possibility is that it could focus on some of the recurring characters from the show but give them a more prominent role. For instance, Castiel could be a main character in this fanfic, and the 'back in black' concept could be related to his journey or a new aspect of his powers.
Obviously Naruto is one of the main characters since it's in the title. But there could also be his friends like Sasuke and Sakura. They might play important roles in understanding why Naruto is holding back and how they can help him overcome this issue.
Typically, Naruto would be one of the main characters. Since it's related to Naruto, he's likely at the center of the story. There could also be his friends like Sasuke, Sakura, Kakashi etc. depending on the direction the fanfic takes. They might play important roles in helping Naruto 'watch his back' against the threats in the story.
Marty McFly and Doc Brown are most likely to be the main characters. They are the central figures in the 'Back to the Future' franchise, so it's expected that they would be the focus in this fanfic set in 2015.
Harry Potter himself is usually a main character. After all, it's a Harry fanfic. Hermione and Ron are also likely to be important characters as they are Harry's closest friends in the original series.
Well, Percy is obviously one of the main characters since it's about him getting sent back in time. There might also be his friends like Annabeth. In the past, he could meet characters who are important in the history of his world, like the ancient gods or heroes. They would play significant roles in his journey back in time as he might need their help or they could be obstacles in his way.
Obviously, Harry and Teddy are the main characters. Harry is a well - known wizard from the original series, and Teddy is his godson.
Definitely Naruto himself. He is the one Konoha wants back, after all. And then there would be his old friends like Sasuke. Sasuke might play an interesting role, either helping Konoha get Naruto back or being a bit hesitant because of his own complex relationship with Naruto. Also, characters like Iruka - sensei, who has always been like a father figure to Naruto, would be important. He could be the one who first realizes how much Konoha needs Naruto and starts the ball rolling to get him back.